1. Field of the Invention
This invention relates generally to apparatus for determining the actual stress history of a drill pipe string and more particularly, but not by way of limitation, to apparatus for electronically detecting and recording the strain on a drill pipe string in a well so that the recorded information can be retrieved and an analysis of fatigue damage to the drill pipe string can be made therefrom after the string has been extracted from the well.
2. Description of the Prior Art
In drilling operations, such as are conducted in the drilling of oil and gas wells, it is necessary to monitor the drill pipe string for fatigue so that failures thereof are reduced or prevented. Presently, such monitoring is performed by making fatigue life predictions based on theoretical calculations of stresses on the drill string and conducting inspections of the pipe in the drill string. These predictions are generally made for drilling operations in deep and highly deviated wells and are generally unreliable.
Because such predictions are unreliable, the visual inspections are performed at arbitrary, and thus sometimes unnecessary, times to avoid unexpected fatigue failures. Such inspections are expensive because of the high cost for making the inspection and also because of the delay in the drilling operation caused by the inspection. To reduce this cost by establishing a proper inspection frequency for the drill string and to better monitor the load, or stress, history of the drill string so that fatigue damage thereto can be better assessed, a more realistic means for measuring the stresses on the drill string is needed.
Several means for monitoring stress with strain gauges associated with a well and means for acquiring data have been proposed. For example, U.S. Pat. No. 3,921,152 in the name of Hagar et al. discloses an automatic data retrieval system which includes strain gauges mounted on pumping wells for providing inputs to fieldlocated remote terminal units which store data and forward data to a central computer upon command from the central computer.
U.S. Pat. No. 3,686,942 in the name of Chatard et al. discloses an apparatus comprising strain gauges for sending electrical signals corresponding to detected strain to electronic circuits for data and information processing.
U.S. Pat. No. 3,376,921 in the name of Manry et al. discloses a plurality of strain gauges positioned on a pipe string with a means connected to a transducer for producing an output signal representative of the relative magnitude of force exerted on the pipe string.
Other proposed means for monitoring loads and stresses on various components in drilling and production operations are disclosed in U.S. Pat. No. 3,864,968, U.S. Pat. No. 4,043,191 in the name of Mills, and U.S. Pat. No. 4,174,628 in the name of van den Bussche et al.
U.S. Pat. No. 3,955,479 in the name of Chapman, III generally discloses electronic circuitry used for obtaining downhole measurements and mounted on an elongated support or chassis removably disposed within a tubular housing.
U.S. Pat. No. 4,184,205 in the name of Morrow discloses a data acquisition system including a multiplexer, an analog-to-digital converter, a microcomputer, read only memory and random access memory.
U.S. Pat. No. 3,495,079 in the name of Halawa et al. discloses apparatus for determining the stresses in a structure due to static and dynamic loading thereof.
Although a number of the foregoing patents disclose various means for monitoring downhole conditions, none of them discloses an apparatus for generating an exact load, or stress, history of the drill string which is retained in the downhole environment until the drill string is extracted from the well. Additionally, these patents fail to disclose that such an apparatus includes data transmitter means and interface means through which communicative access to the data stored within the apparatus is gained by an external device.